As far as I am aware it has one FPL53 element between two crown & flint elements.

The TOA has a unique design : a cooke triplet 2 x FPL53 1x BS7 and a wide air space. Thanks to this design it has almost not any spherochromatism. This is the same for te TOA150.

This is the best performer among the 130 for photographic use especially if you would kike to do some CaK sun picture: the strehl at 400nm will be much higher than any other triplet of the market of simillar F/L

At little bit overkill for visual use, and its heavy and take time to cool down.

At My side I prefered the AP130GT, much smaller, shorter F/L, faster cool down, very good for APS-C sensor or smaller, exclent for wide field visual use.

But the Toa has a better optic for photo use, wider corrected spectrum, widder corrected field thanks to its 4" focuser and flatener.

You can see here test reports and have a look to the spherochromatism graph. The flatener test is also excelent at the Tak side, showing a very low focus shift on a wide field: it is diffraction limited at least at 27mm from the center and thus over the full visual spectrum. The strehl is still higher than 0.92 in the blue channel, where other classical triplet usualy give about 0.7. In the violet/UV channel, the difference is even greater.

This statement begs the question: So how much then does the eyepiece influence chromatic aberration? (Given a pure image, because it was reflected by a mirror, does it remain pure going thru 3 or more elements in an eyepiece?)

Reflective optics have their own set of aberrations and difficulties. Personally, I find refractive optics are best suited for small scopes, reflective optics for larger scopes. Each represents compromises, optimizations.

The answer is yes eyepieces introduce their own CA. In "Telescope Optics" by Rutten and VanVerooj they raytrace a few designs for three colors and clearly show how eyepieces behave from a CA standpoint.

The answer is yes eyepieces introduce their own CA. In "Telescope Optics" by Rutten and VanVerooj they raytrace a few designs for three colors and clearly show how eyepieces behave from a CA standpoint.

You say that many of the scopes you have looked though are color free. What you are ignoring is the response of the detector, your eye. Eyes are notoriously insensitive to wavelengths away from yellow-green, particularly red and blue. Everyone's eyes are different. However if you can't see color using triplets, you must also allow that some folks don't see it with doublets. I was only saying that when you look at a cold, hard ray trace analysis neither triplets nor doublets are color free.

As it relates to a one shot color camera, all CCDs are monochrome. To make a "color" camera the manufacturer inserts red, blue, and green color filters between the source and the ccd. So what you are recording is a pseudo-color image based on a summation of adjacent red, green, and blue pixels. And since you are only looking at three specific wavelengths it is possible for it to be color free. More likely is that the red and blue signals are just much weaker than the green and therefore don't show up in the final image.

If any of you feel this is a stupid argument, I ask that you try to understand that the original intent was to understand the differences between the two and to get better informed.... Many people on here really don't understand what some of the glass types are and how they're arranged affects performance, let alone trying to grasp the, what seems to ne, loosely defined APO designation

If you have nothing empirical to add other than to express your opinion, try to make it constructive so that the readers can benefit somehow

Thanks Rodger. Actually we agree. When I owned a Brandt 8" doublet F/13 achromat andthe color was "impressive". White stars appeared yellowish to me. After a while using the scope white stars appeared white to me! Many people would look through one of today's modern doublets and what little color they saw might "disappear" in time. Plus many people are far more color sensitive than others. My feeling is that if I see no extraneous color at the eyepiece the lens is color free. Of course I also feel if the double star companion of Antares looks green to me it's green (although we know it is blue and looks that way as a CONTRAST effect).
So, my opinion, based on almost 48 years of observing is that many of today's doublets, and I will use the Takahashi FS series (Fluorite) as an example, are as color free as you need for visual observations and they get more and more color free the longer you use them. Today's triplet apos like the Takahshi TOA and TSA models along with my favorite, Astro-Physics (and others)represent the height of the maker's art. They don't get any better. Is it necessary to own a triplet apo if you want a fine telescope to observe with? No. If you want the very best example of what can be done today I would say yes. Just my opinion based on experience.
I hope this is helpful.

I believe that is what I am attempting to do. Empirically some people don't see color in doublets or triplets. Some do. When you look at it from a strict analytical standpoint, neither are color free.

I think there is more to it than just whether a scope is a doublet or a triplet, more to it than whether a scope uses FPL-53 or FPL-51.... The aperture and the focal ratio are important in determining the level of color correction in apo/ED scopes just as they are in determining the color correction in achromats and singlets.

Concerning color correction there are 2 differences between triplets and doublets.

- ED triplets have lower spherochromatism than ED doublet of same diameter and same focal ratio. This means triplets are corrected on a wider spectrum, at least from blue to red and sometimes from violet to deep red, where ED doublet are corrected from light blue to light red.

All the refractors are optimized (i.e. "centered")at a single color, usually the green, or yellow-green. The red and blue channels are less corrected (one is over corrected and the other under corrected)

There is an exception : the takahashi TOA series that have almost no spherochromatism. The blue is as good corrected as the red and green.

For visual use, ED doublets are usually good enough, but the sperochromatism still quickly rises together with the diameter.

For CCD that are sensitive on a wider spectrum than eyes, the triplet makes more sense.

At the end, a triplet of same F ratio and same diameter and same ED glass focuses more energy in the diffraction pattern than a ED doublet resulting in an higher contrast.

The difference is marginal for long focal ratio and small diameter, like the TSA102 vs the FS102 for instance, but for larger diameter and shorter focal ratio, the triplet can take the advantage even for visual use.

Other than achros, apos produce much less longitudinal chromatic aberration. OTOH, as a result of their usually much shorter focal lengths they tend to be plagued by spherochromatism (=variation of spherical aberration with wavelength), first of all.
As can be seen on Wolfgang Rohr's Website "astro-foren.de", despite the TOA's virtually complete freedom from spherochromatism, they are OTOH, as far as longitudinal chromatic aberration is concerned, not better color-corrected than other top-notch apochromats! - Yes, they are good performers in that latter discipline as well, but with an RC-index of ~0.5 are no better than Tak-TSAs, for example...
Plus in this contex, their Strehl is distinctly lower in blue as the ones in green, yellow and red.

If any of you feel this is a stupid argument, I ask that you try to understand that the original intent was to understand the differences between the two and to get better informed.... Many people on here really don't understand what some of the glass types are and how they're arranged affects performance, let alone trying to grasp the, what seems to ne, loosely defined APO designation

If you have nothing empirical to add other than to express your opinion, try to make it constructive so that the readers can benefit somehow

Clear skies!

Joe,

I too continue trying to understand this subject. Call me old-school but I believe in defining a word and then sticking to that definition. Once you start chipping away at or broadening a definition, the next thing you know the original word no longer means anything at all. That being said, I am of the understanding that the original definition of apochromatic was established by Abbe and that (in addition to spherical aberration and coma) it requires there be correction for 3 widely spaced wavelengths...i.e. bringing three colors to focus at the same point.

So the way I look at answering this question of "can a doublet be apochromatic" simply comes down to answering the question of "can two pieces of glass accomplish bringing 3 widely spaced wavelengths to focus at the same point while correcting for spherical at 2 and coma as well."

If the answer is Yes, then a doublet has the ability to be apochromatic.

If the answer is No, then they can't.

As an aside, I'm also not sure that bringing three colors to focus necessarily means color-free....does it? Or is that just a convention we're assuming to be meant by apochromatic but really isn't necessarily the case?

Don't know where the Rohr's numbers come from, but Takahashi China says 130TOA has 0.01mm focus shift between g and C lines (F to C is only slightly less). That's like 1/50 of the C-F shift in an achromat. The graph they show implies more like 0.02mm, but it's still far in the apo land.

In this other report from AiryLab / France you can see on p12, that the focus shift between the 3 colors is 60µ. Very similar to concurent products. The TOA has a little bt longer Fl ratio which helps in containing the longitudinal chromatic error.

Well, what I see is that Rohr gives the relative defocus vs. e-line going from -68 microns in F to +32 microns in the C line. That's 100 microns, or 0.1mm, at least five, according to the graph, or ten, according to Takahashi China, times the TOA's advertised shift. If we look at the graph, the entire range is within less than 0.02mm. Can Rohr control measurements to that level of precision? He doesn't specify how does he determine best focus position. What I've seen of his site - admittedly, not much - doesn't make me confident. Takahashi Europe shows this same TOA130 chromatic shift graph. Chromatic correction is not subjected to significant changes vs. design, as long as it uses design glasses. That includes spherochromatism. E-line correction will be less good, but it is not a factor affecting secondary spectrum. Rohr's test is in disagreement with the official data, and if I am to take my pick, it's Takahashi. Still remember the mess that Rohr made of testing a simple system such as Dall-Kirkham (Tak Mewlon). The other testing site, the French one, didn't impress me either. Someone claiming he's from the site didn't show he knows why the Zernikes weren't used properly, or how their longitudinal aberration graph was obtained (CN thread). These days, sophisticated optics software is readily available, and may get used by those not quite up to the task.

Chromatic correction is not subjected to significant changes vs. design, as long as it uses design glasses.

I should have said as long as it uses design glasses and have power components of the objective near optimally balanced. If, for instance, the positive component is weaker, the blue end shifts away, and the red end comes closer. Still, this means that the error is induced to both ends, if starting from near-optimum balance (design).

On the other hand, the info on the Tak is not consistent as well, although not nearly as much. Confusion seems to be coming from vendors' interpretations and assumptions, but part of it is that the official info from Takahashi is scarce and incomplete. I am not aware of any glass combinations that would produce nothing even close to such a tight chromatic shift curve in a this large/fast triplet. The possibility is that it shows chromatic shift based on best, not paraxial (usual) foci. But one can only guess. It's kind of strange that people do not get specific info on anything that they are paying good money for.

The other testing site, the French one, didn't impress me either. Someone claiming he's from the site didn't show he knows why the Zernikes weren't used properly, or how their longitudinal aberration graph was obtained (CN thread). These days, sophisticated optics software is readily available, and may get used by those not quite up to the task.

Vla

Stange... the guy is a professional guy with profesionnal tools, he knows his job.
Whatever the risk of error on the focus plane measurement is very low.

But to me the most intresting in these test reports are not the absolute values, but the relative values from one scope to another tested in the same conditions by the same operator.

You mean the spherochromatism graph calculated from 3,5 and 7...The graps seems a little bit strange to me at higher values than 70/80%, especially if we compare to the front wave map, but however, this graph is not so far from what is expected : for instance we can see a 70/80% crossing and the graph is roughly OK.I don't know whether the "difference" at high values comes from the calculation hypothesis or from the real world difference to ideal optics/specs, but at the end this gives a good idea of how the optics work.To me the most interesting is comparing the graphs from a refractor to another.

The second test from Rohr gives 70µ offset between bue and yellow which seem to be confirmed by the color foucault, no?

No. What I mean is:
1-arbitrarily discarding Zernike terms, which shows no knowledge of how they work
2-mixing pupil size with the value of Zernikes, which is a factor in ophthalmology, not in telescope optics
3-evaluating primary spherical based on its Zernike term, which has little sense with apos where it is only a component in modeling the wavefront, routinely affected by the higher order terms, as well as other aberration terms
4-saying that the wavefront *phase* error is given in microns (that's zero level error)
5-saying that TOA and Maks are the only systems *not showing any spherochromatism* (another zero-level error)
6-not answering the obvious inconsistency between his explanation of how the LA graph was obtained and their wavefront maps
7- saying that in the AP155 5th and 7th order spherical (i.e. 6th and 8th in the modern nomenclature) are "quite the same over the spectrum" (take a look of the graph)

The second test from Rohr gives 70µ offset between bue and yellow which seem to be confirmed by the color foucault, no?

Not really. The second test is for the 150/1100 TOA, with 10% longer f.l. which means that the secondary spectrum should be 10% greater only based on the f.l. And somewhat more than that due to secondary spectrum in apos being affected by the magnitude of spherochromatism, which increases exponentially with the f-ratio (f/7.3 vs. f/7.7). With the TOA130 tested at 100 microns F-C, this one should be more than 110 microns. Instead, it is 68 microns, 2/3 of the smaller/slower TOA's. Doesn't add up.

Okay you have issues with some of the testing Rohr does. But as far as I can tell he is the only source for in-depth testing of astronomical optics on the net. I am interested in what he does because when I want to buy a telescope (new or used) I can go to his website and find out how well the optic was made. And if I remember he also gives the polychromatic strehl for refractors which I don't see very often. No one, and I mean no one, does more exhaustive and thorough testing of telescope optics. Maybe you should give him some competition?

Okay you have issues with some of the testing Rohr does. But as far as I can tell he is the only source for in-depth testing of astronomical optics on the net. I am interested in what he does because when I want to buy a telescope (new or used) I can go to his website and find out how well the optic was made. And if I remember he also gives the polychromatic strehl for refractors which I don't see very often. No one, and I mean no one, does more exhaustive and thorough testing of telescope optics. Maybe you should give him some competition?

Or maybe some advice...

Testing is one thing but it must be done properly and with an understanding of the analysis.